In recent times, especially in the electronic field, the printing of electrically conductive lines (traces) onto flexible substrates has become a relevant solution for the implementation of Printed Circuit Boards (PCBs) of the flexible type (Flexible Printed Circuits, FPCs). This solution is replacing, at least in some applications, the traditional technologies based on the use of copper and/or aluminium.
In comparison e.g. with the techniques of copper etching, one of the advantages which may be achieved through printing is the high versatility and flexibility in adapting to possible changes.
As a matter of fact, printing enables the introduction of new circuits and/or circuit layouts on an FPC support without relevant investments. Moreover, such changes may be implemented even directly by the electronic assembler, without the need of involving the FPC supplier, who otherwise would find himself in the position of adapting process steps (chemical processes such as etching, masking, photolithography etc.), with the consequent logistics and cost issues, in order to take the modifications into account.
These aspects may be particularly meaningful in the field of Solid State Lighting (SSL), wherein both products and sources (e.g. LED sources) are constantly and rapidly evolving.
The electrical conductivity of inks and other conductive materials (e.g. pastes) which may be used for printing e.g. onto an FPC, however, is still lower than the conductivity of traditional materials such as copper or aluminium, which are used in conventional circuits.
This may be a relevant limitation especially as regards flexible (“flex”) linear LED modules, having a plurality of units connected in parallel, the maximal length of the flexible LED module being a function of the electrical resistance of the conductive lines or traces: the connection of a plurality of units implies a higher current intensity, with a corresponding increase of the voltage drop across the module.
The flexible linear LED modules which employ FPC circuits may be implemented as a series of Single Electrical Units (SEUs) connected in parallel along a ribbon-shaped support, i.e. a strip. The electrical connection among the various SEUs may be obtained e.g. through two conductive lines adapted to act as electrical distribution lines (e.g. acting as positive voltage, V+, and negative voltage, V−, supply lines) extending along the strip.
The number of such electrical distribution lines may be even higher if there is the need of exchanging driving signals (e.g. to perform a dimming function) or feedback signals (e.g. about the thermal behaviour of the sources) with the light radiation sources.
In this respect, a solution has been proposed wherein the electrical distribution lines (which may be present in a number of two or more) exhibit low ohmic resistance, e.g. thanks to the use of a material with low resistivity such as copper or aluminium, and/or through imparting a wide section area to such lines or traces, in such a way as to reduce resistance.
If a change of the circuit layout is needed, said solution imposes a nearly complete readjustment of the FPC structure, involving therefore the FPC supplier and leading to the previously mentioned logistics and cost issues.
Moreover, said copper/aluminium lines are normally available in standard thicknesses, e.g. amounting to 35-70-105 μm (1 μm=1×10−6 m).
The use of a higher thickness leads to a decrease of line resolution, which imposes limits as regards small-sized components (e.g. LEDs). Another problem is that higher thicknesses affect the module flexibility/bendability, which are key features in a flexible module.
If, in order to increase the conductive cross-section, the width (instead of the thickness) of the lines is increased, this affects the size of the final product, which may be inconvenient for the end user.
Another solution proposed for elongate flexible LED modules consists in adding a second layer to the FPC component, so that the electrical distribution lines (+ and − and optionally driving signals) are arranged on the bottom layer and are connected to the top layer through electrically conductive vias.
In this way, the top layer hosts (only) the lines adapted to connect the vias with the circuits. Also in this case, if changes must be made, this solution imposes a re-designing and/or a rearrangement of the FPC component, requiring the intervention of the manufacturer thereof, at least as regards the top layer.